The present invention relates generally to biological tissue stimulators and relates more particularly to biological tissue stimulators having high-voltage output circuits.
Biological tissue stimulators are known to be medically useful. In one example, transcutaneous electrical nerve stimulators (TENS) are utilized to mask pain signals in a human body before they reach the brain giving the subject apparent relief from pain. In such TENS devices, electrical pulses, usually current pulses of a selected rate, amplitude, pulse width and duty cycle, are delivered to the skin of the subject by one or more electrodes. The timing characteristic of the delivered pulses may be predetermined, as for example, by the prescribing physician and/or may be individually selected or controlled by switches available to be operated by the subject. Additionally, individual parameters, or even entire pulse programs, can be varied in a predetermined or random basis by the TENS device itself.
Another example of useful biological tissue stimulators are neuromuscular stimulators (NMS) which can be utilized to electrically stimulate muscle activity of a patient. In such neuromuscular stimulators electrical pulses, again probably current pulses of a carefully controlled rate, amplitude, pulse width and sequence are delivered by electrodes to a site or sites near the muscle to be stimulated in order to activate or contract the muscle. The initiation and control of such sequence of pulses may be patient controlled.
In both of these biological tissue stimulators, an output stage supplies electrical pulses having certain timing and amplitude characteristics. The amplitude of these electrical current pulses is usually of a substantial, e.g., 10-100 milliamperes, current level. Such current levels are achievable by means of a high-voltage power supply, e.g., 40 to 100 volts. The logic circuits necessary for supplying the timing and amplitude information are typically powered by a low-voltage level, e.g., 2.7 to 5 volts. Thus, in order to both service the high-voltage output circuits and the low-voltage logic circuits, at least two separate voltage levels, i.e., sources of electrical power or energy, must be maintained.
Since the biological tissue stimulators are designed to be utilized with a human subject, small size and independence from external power sources are strongly desired. Small batteries are commonly utilized as a source of electrical energy. Since sources of electrical energy at several voltage levels are required, either batteries of several different voltages are required, or the voltage from one set of batteries must be either stepped down or stepped up to obtain the additional voltage levels. The smaller size requirements and operating cost constraints for biological tissue stimulator mitigates against the use of batteries with a significantly higher voltage, e.g., 22.5 to 90 volts. However, the use of batteries of a lower voltage level, e.g. 1.2 to 9 volts, necessitates additional logic and control circuitry to achieve the stepped up voltage with appropriate regulation. These additional components also mitigate against small size due to their number and the increased size of the energy source (batteries) required because of the additional power they consume.